Joseph Louis Gay-Lussac (1778-1850), French physical chemist. Gay-Lussac was born at St. Leonard de Noblat in central France. He was educated at home before being sent to Paris in 1797 to study at the Ecole Polytechnique. In 1804 he made balloon ascents to measure changes in magnetism and air composition with altitude. In 1808 he published the law of combining volumes. This states that the volumes of gases that react with one another, or are produced in a chemical reaction, are in the ratios of small integers (whole numbers). In 1809 he was appoointed professor of chemistry at the Ecole Polytechnique, and served as professor of physics at the Sorbonne. His work gave support to Dalton's atomic theory, and formed the basis for Avogadro's law. Collaborating with Thenard, he was the first to isolate the element boron, and studied the newly-isolated elements sodium, potassium, and iodine.